The objective of this project is to study the molecular events that lead to mutation in the bacterium E. coli after induction of the SOS-response. The error-prone type of DNA replication that is presumed to be responsible for SOS-mutagenesis will be studied in an in vitro replication system. The accuracy with which crude extracts of E. coli cells copy normal or damaged single-stranded bacteriophage M13 DNA will be used as an indicator for in vitro SOS-expression. Characterization of the components involved is important for the study of SOS-mutagenesis and for the question of the regulation of mutation rates in general.